ISSN 2398-2993      

Subacute ruminal acidosis


Owen Atkinson

Keshab Batajoo

Dairy Veterinary Consultancy Ltd logo

Synonym(s): SARA


  • Subacute ruminal acidosis (SARA) is one of the most important rumen health problems observed in intensively managed dairy cattle.
  • SARA is characterized by repeated bouts of low rumen pH, but unlike the situation with acute acidosis, the pH recovers after each bout. These bouts of low pH typically last for several minutes to several hours.
  • Cause: SARA can be induced experimentally by either:
    • Feeding insufficient fiber, or fiber which induces insufficient chewing or ruminating.
    • Feeding excess starch (cereals), particularly if overprocessed and/or heat treated which makes it very quickly fermentable in the rumen.
  • Of the two methods, research suggests it is the latter which seems to cause the greatest ill-health effects.
  • Signs: reduced or cyclic feed intake, decreased milk production, reduced milk fat (this is a poor indicator), poor body condition score despite adequate feed intake Approach to the underweight cow, loose feces/unexplained diarrhea, high rates of culling or unexplained deaths and sporadic cases of caudal vena cava syndrome (may present as epistaxis).
  • Diagnosis: SARA is a herd condition and is often diagnosed without due care. Diagnosis can be challenging and may be undertaken in one of two ways:
    • Assessing rumen fluid samples, collected either by rumenocentesis Rumenocentesis and rumen fluid analysis (by needle), or stomach tube. A minimum of 12 cows should be tested.
    • Data collected from telemetric rumen pH boluses sensors, which are inserted in a representative sample of the herd (one per 30 cows is suggested).
  • Treatment: no specific treatment, rather secondary conditions may need to be treated and the focus should be on prevention.
  • Prognosis: variable.
Print off the farmer factsheet on Subacute ruminal acidosis (Farmer Factsheet) to give to your clients.



  • Feeding cows with a highly fermentable starch diet provides the energy precursors for high milk production, but the risk of SARA increases with quantity of concentrates fed.
  • Rumen fermentation of feed produces volatile fatty acids (VFA's: acetate 60-65%, butyrate 5-10%, propionate 15-20% and some others) Rumen function and fermentation. These are the main energy source for cows, and are normally absorbed into the blood through the rumen wall at a similar rate as they are being produced.
  • If the surface area of the rumen is not developed enough, the VFA’s do not get absorbed rapidly enough, so the acids build up in the rumen and cause acidosis.
    • The normal (healthy) rumen operates at between pH 5.6 and 6.5.
    • Below pH 5.5 the rumen microorganisms cannot survive and atypical rumen organisms are favored (which produces stronger acids and lower the pH even more).
  • Feeding some starch pre-calving increases the surface area of rumen papillae, so insufficient exposure of the rumen to starch pre-calving, or too-fast introduction of starch after calving, can both lead to a greater amount of VFA production than can be absorbed through the rumen wall.
  • There are three principle ways that SARA reduces fertility:
    1. Cows with SARA (poor rumen health) have poor populations of rumen microorganisms. This means the rumen is not working efficiently and cannot digest the fiber which is an important energy source. The cow goes into more severe negative energy balance.
    2. Cows with SARA stop eating, sometimes for a short period while the rumen recovers, and subsequently have reduced energy intake.
    3. Cows with SARA have greater amounts of starch which by-pass the rumen. Whilst the cow can cope with small amounts of starch by-passing the rumen and being digested in the small intestine, large amounts undergo secondary fermentation, causing over-growth of unfavorable bacteria in the hind gut and production of lipopolysaccharides (LPS). LPS’s can act as toxins and have a direct effect on ovaries, reducing egg quality and hormone production.
  • A negative energy balance results in loss of body condition and negatively impacts on fertility.

Predisposing factors

  • The risks of poor rumen health are greater with: 
    • High concentrate feeding.
    • Not enough fiber in the diet (NDF), or physically effective fiber (peNDF) which stimulates cud chewing.
    • Diet sorting – leading to insufficient fiber intakes for some cows.
    • Poor transition cow management (poor rumen acclimatization) Feeding the dry cow.
    • Insufficient forage intakes (physically effective fiber) – such as when there is competition at troughs/ insufficient feedspace/ poor palatability of forage.
    • Heat stress (less time spent ruminating).
    • Poor cow comfort: lower lying times (less rumination).
    • “Slug feeding”:
      • Large amounts of concentrate feeds may be fed, usually in the parlor.
        • This occurs when Total Mix Ration (TMR) diets are first fed out, as some cows will sort this and select the concentrates.
        • Lame cows may also tend to have fewer but larger meals, which are effectively slug feeds.
      • For example, two cows might have the same overall fiber and concentrate levels in their diet, but the cow which has her meals spread throughout the day will produce VFA’s at a steadier rate. These VFAs are absorbed into the blood stream as they are produced. The rumen pH fluctuates but is maintained above 5.5. Conversely, the cow which has her daily intake in two main meals (“slug feeds”) has periods of very rapid VFA fermentation, which outstrips the ability of the cow to absorb them into the bloodstream. This cow’s rumen drops below pH 5.5 on two occasions, indicating periods of SARA. Her rumen microorganism population will be less healthy and her ability to fully utilize the diet will be reduced due to less efficient fermentation. More undigested feed particles are likely to bypass the rumen, which not only decreases available energy, but increases the risk of secondary fermentation and LPS production.


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Further Reading


Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Batajoo & Shaver (1998) In situ dry matter, crude protein and starch degradabilities of selected grains and by-product feeds. Animal feed Sci Tech 71, 165 – 176 Academia.
  • Batajoo K K & Shaver R D (1994) Impact of nonfiber carbohydrate on intake, digestion and milk production by dairy cows. J Dairy Sci  77 (6), 1580-8 PubMed.
  • Weidner S J Grant R J (1994) Soyhulls as a replacement for forage fiber in diets for lactating dairy cows. J Dairy Sci 77 (2), 513-521 PubMed.

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